1. Field of the Invention
The present invention relates in general to solid oxide electrolytic devices, including solid oxide fuel cells (SOFC's) and oxygen generation systems (OGS') and novel interconnect structures in such devices. In particular, the invention relates to the use of chrome-containing alloys in these devices, and the use of protective layers deposited to prevent corrosion, degradation, and/or increased electrical resistivity of the alloys.
2. Description of the Related Art
Solid state devices based on high-temperature (>600° C.) solid oxide electrolyte behavior have become increasingly important for a variety of applications. Such devices are of interest as viable options for power generating fuel cells, as well as for producing pure oxygen, hydrogen, and other such gases that may be produced through dissociation of oxygen-bearing gases.
It has been found in the prior art that chrome and chrome-containing metallic alloys, referred to variously as superalloys, bimetals, “Met-X”, Siemens “Plansee”, “CRF” and the like, provide an economical and suitable material for the metallic components of such solid oxide electrolytic devices as solid oxide fuel cells (SOFC's) and oxygen generators. Problems exist, however, in preventing the chrome in such alloys from diffusing and/or becoming oxidized in the high-temperature (>800 C), oxygen-rich environments common to such devices. Degradation of the device structure, due to chrome oxidation and/or diffusion, can result in failure of the device, due to failure of an integral seal, an increase in internal resistance, or contamination of device components.
It has been found in the prior art that use of certain electrically conductive multicomponent oxides, typically of a defective perovskite structure, can be deposited on these alloys to form a diffusion barrier that blocks, at least initially, sublimation of Cr into the gaseous environment of the device. Also, metals may be deposited onto the Cr alloy component, the component annealed in oxygen environment to form a conducting oxide phase containing Cr, wherein the Cr-containing oxide phase is then found to be an effective diffusion barrier, as well. This latter approach is taught in U.S. Pat. No. 6,054,231 to Virkar and England. In Virkar, the proposed use of such a chrome-containing barrier layer was to essentially act as a sink for trapping the Cr as it diffused out of the Cr-based alloy component. However, since the oxide phase of Virkar does not actually stop Cr diffusion, Cr eventually diffuses to the gas/solid interface, where it can sublime, albeit, at a slower rate than were the oxide barrier layer not present. This latter sort of a barrier is not an ideal solution for stopping degradation in the relevant devices, since it does not stop Cr diffusion, but only impedes it. At the same time, the use of these thick-film, multi-component oxides present complex reproducibility issues. In part, it is found in the present invention that such thick-film barrier thicknesses (>10 um), as well as their suitable deposition methods, will tend to result in a coating/substrate system that is not mechanically sound, and will result in fracturing and stress-induced diffusion across the thick films of these prior art accounts. These conducting oxides were previously deposited by methods that provide quite thick films, usually of thicknesses greater than 25 um, in order to provide a sufficiently long lifetime for device operation. As reported by workers using such an approach, this proved to be quite expensive in materials usage.
Additionally, the formation of such defective oxides that contain chrome oxide have been found, in the present invention, to result in a material that can be easily modified at its surface in the relevant device environments. The combination of high temperatures and electrical fields found in SOFC's and OGS' devices can readily alter the valence states existing at the surface of such electrically conducting oxides, so that various reduction and diffusion processes are activated, resulting in eventual degradation of the diffusion-barrier quality of the oxide material.